The present invention relates to a light guide plate used for liquid crystal display devices and the like.
Liquid crystal display devices use a backlight unit (planar lighting device) for radiating light from behind the liquid crystal display panel to illuminate the liquid crystal display panel. A backlight unit is configured using a light guide plate for diffusing light emitted by an illumination light source to irradiate the liquid crystal display panel and optical parts such as a prism sheet and a diffusion sheet for rendering the light emitted from the light guide plate uniform.
Currently, large liquid crystal televisions predominantly use a direct illumination type backlight unit that comprises a light guide plate disposed above an illumination light source. This type of backlight unit comprises a plurality of cold cathode tubes serving as a light source provided behind the liquid crystal display panel whereas the inside of the backlight unit provides white reflection surfaces to ensure uniform light amount distribution and necessary luminance.
To achieve a uniform light amount distribution with a direct illumination type backlight unit, however, a thickness of about 30 mm in a direction normal to the liquid crystal display panel is required, making further reduction of thickness of the backlight unit difficult using the direct illumination type backlight unit.
Among backlight units that allow reduction of thickness thereof, on the other hand, is a backlight unit using a light guide plate in which light emitted by an illumination light source and entering the light guide plate is guided in given directions and emitted through a light exit plane that is different from the plane through which light enters.
There has been proposed a backlight unit of a type using a light guide plate in the form of a plate containing scattering particles for diffusing light mixed therein and formed into a transparent resin, whereby light is admitted through the lateral faces of the plate and allowed to exit through the top surface.
JP 07-36037 A, for example, discloses a light diffusion light guide light source device comprising a light diffusion light guide member having at least one light entrance plane region and at least one light exit plane region and light source means for admitting light through the light entrance plane region, the light diffusion light guide member having a region that has a tendency to decrease in thickness with the increasing distance from the light entrance plane. JP 07-36037 A, for example, discloses a light diffusion light guide light source device comprising a light diffusion light guide member having at least one light entrance plane region and at least one light exit plane region and light source means for admitting light through the light entrance plane region, the light diffusion light guide member having a region that has a tendency to decrease in thickness with the increasing distance from the light entrance plane.
JP 08-248233 A discloses a planar light source device comprising a light diffusion light guide member, a prism sheet provided on the side of the light diffusion light guide member closer to a light exit plane, and a reflector provided on the rear side of the light diffusion light guide member. JP 08-248233 A discloses a planar light source device comprising a light diffusion light guide member, a prism sheet provided on the side of the light diffusion light guide member closer to a light exit plane, and a reflector provided on the rear side of the light diffusion light guide member. JP 08-271739 A discloses a liquid crystal display comprising a light emission direction correcting element formed of sheet optical materials provided with a light entrance plane having a repeated undulate pattern of prism arrays and a light exit plane given a light diffusing property. JP 11-153963 A discloses a light source device comprising a light diffusion light guide member having a scattering power therein and light supply means for supplying light through an end face of the light diffusion light guide member.
Also proposed in addition to the above light guide plates are a light guide plate having a greater thickness at the center thereof than at an end thereof at which light is admitted and at the opposite end; a light guide plate having a reflection plane inclined in such a direction that the thickness of the light guide plate increases with the increasing distance from a part of the light guide plate at which light is admitted; and a light guide plate having a configuration such that the distance between the front and rear plane is smallest at a location at which light is admitted and that the thickness of the light guide plate is greatest at a greatest distance from the location at which light is admitted (See, for example, JP 2003-90919 A, JP 2004-171948 A, JP 2005-108676 A, and JP 2005-302322 A). Also proposed in addition to the above light guide plates are a light guide plate having a greater thickness at the center thereof than at an end thereof at which light is admitted and the opposite end, a light guide plate having a reflection plane inclined in such a direction that the thickness of the light guide plate increases with the increasing distance from a part of the light guide plate at which light is admitted, and a light guide plate having a configuration such that the thickness of the light guide plate is greatest at a greatest distance from the location at which light is admitted (See, for example, JP 2003-90919 A, JP 2004-171948 A, JP 2005-108676 A, and JP 2005-302322 A).
While a thin design may be achieved with a tandem type backlight, for example, using a light guide plate of which the thickness decreases with the increasing distance from the light source, such a backlight unit yielded lower light use efficiency than the direct illumination type backlight unit because of the relative dimensions of the cold cathode tube to the reflector. Further, where the light guide plate used is shaped to have grooves for receiving cold cathode tubes, although such a light guide plate could be shaped to have a thickness that decreases with the increasing distance from the cold cathode tube, luminance at locations above the cold cathode tube disposed in the grooves increased if the light guide plate is made thinner, thus causing uneven luminance on the light exit plane to stand out. In addition, all these light guide plates posed another problem: a complex configuration leading to increased machining costs. Thus, a light guide plate of any of such types adapted to be used for a backlight unit for a large liquid crystal television having a screen size of say 37 inches or larger, in particular 50 inches or larger, was considerably expensive.
JP 2003-90919 A, JP 2004-171948 A, JP 2005-108676 A, and JP 2005-302322 A propose light guide plates growing thicker with the increasing distance from the light entrance plane to achieve stabler manufacturing or to limit luminance unevenness (unevenness in light amount) using multiple reflection. These light guide plates, made of a transparent material, allow light admitted from the light source to pass and leak through the opposite end and therefore need to be provided with prisms or dot patterns on the light reflection surface thereof.
Also proposed is a method whereby the light guide plate is provided with a reflection member near its light entrance plane on the opposite side from the light entrance plane to cause admitted light to undergo multiple reflection before allowing the light to exit through the light exit plane. To achieve a large light exit plane with these light guide plates by this method, however, the light guide plate needs to have an increased thickness, which increases weight and costs. Further, the light sources are projected into the light guide plate and perceived as such to cause uneven luminance and/or uneven illuminance.
On the other hand, the side light type backlight unit using a flat light guide plate contains fine scattering particles dispersed therein in order to efficiently emit admitted light through the light exit plane. Although such a flat light guide plate may be capable of securing a light use efficiency of 83% at a particle density of 0.30 wt %, its luminance dropped in an area about the center as illustrated by the illuminance distribution indicated by a solid line in FIG. 15 when it was adapted to provide a larger screen despite scattering particles evenly dispersed therein, thus allowing uneven luminance to stand out to a visible level.
To even out such uneven luminance, the density of the scattering particles needed to be reduced in order to increase the amount of light leaking from the area about the center, thus reducing the light use efficiency and the luminance. For example, when the density of the scattering particles was 0.10 wt %, with the other conditions being equal, the luminance decreased and the light use efficiency lowered to 43%, although uneven luminance could be evened out considerably, as illustrated by a dotted line in FIG. 15.
A large display such as a large liquid crystal television is required to present a luminance distribution on the light exit plane that is bright in an area close to the center of the screen as compared with the periphery (edges) thereof, i.e., a convex curve distribution such as a distribution representing a bell curve. Although a flat light guide plate containing scattering particles dispersed therein may be capable of providing a flat luminance distribution by reducing the density of the scattering particles, it is incapable of achieving a convex luminance distribution.
It has also been proposed to use a light guide plate having a thickness that, conversely to the tandem type, increases with the increasing distance from the light source for a thin backlight unit. Although use of such a light guide plate does achieve a thinner design and a flat luminance over the whole screen, such a proposal did not provide any teaching or did not give the slightest consideration as to how one may achieve a convex luminance distribution whereby an area close to the center of the screen is brighter than the periphery thereof as required of thin, large-screen liquid crystal televisions.
Further, although there has been a demand for a yet thinner design in a large display such as a large-screen liquid crystal television, there has not been made any proposal nor has any teaching been provided as to how one may achieve emission of light with a high light use efficiency, a reduced level of unevenness in luminance, and a convex luminance distribution with a thickness comparable to that of a sheet light guide plate or a so-called light guide sheet.